Dr Moore answers: My rule of thumb is that 90 percent of mothers will have some type of back pain for six weeks to six months postpartum. During pregnancy, your placenta secretes hormones, which cause laxity of the ligaments in your back. This allows lordosis, that big arch in the small of your back, which helps you support the weight of a sack of concrete of the front of your belly. After delivery, it takes time to re-tighten and strengthen them. Unfortunately, instead of resting, you now spend 24/7 bending over to lift a 10-pound baby out of cribs and car seats. Take care of your back. You only get one.